Salt rejection behaviors coupled with membrane fouling in seawater reverse osmosis (SWRO) processes were studied through a lab-scale membrane fouling test. Seawater was taken from Masan Bay, South Korea and foulants such as humic acid (HA), sodium alginate (SA), and silica nanoparticle (S120; particle diameter = 120 nm), were spiked to the seawater for the rapid fouling test. All the test conditions except model foulant were controlled equally. Salt concentration in permeate increases with flux decline even with no fouling because of increased water permeation and relatively constant salt passage. Salt mass transfer can be altered in the presence of fouling layer. Organic foulants such as HA and SA tend to build up denser fouling layer than S 120 nanoparticles. Thus, S 120 fouling layer induced cake enhanced concentration polarization (CECP) to accelerate salt rejection decreasing rate while HA and SA fouling layer hindered the convection of salt to lead cake reduced concentration polarization (CRCP) decelerating the rate. These combined effects of flux decline, CECP, and CRCP can be the causes of the coupled behaviors of permeate flux and salt rejection.